Mercedes W204 C220 CDI oil cooler seal failure — coolant mixing
Mayonnaise-like emulsion on the oil filler cap and dipstick. Coolant level dropping slowly. The oil cooler mounted on the side of the OM651 engine block uses rubber O-ring seals that harden and fail. This allows engine coolant to mix with engine oil. If caught early, replacing the oil cooler seals and flushing the cooling and lubrication systems is sufficient. If ignored, bearing damage and head gasket failure can result.
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